Section Width is the width of an inflated tire in millimeters at its widest point from sidewall to sidewall on most cars, SUVs, CUVs and light trucks. For other tires, called flotation sizes, the width is measured in inches and are mostly used on light trucks. These tires usually contain a decimal point in the measurement.

Aspect Ratio is the dimensional relationship of the section height to the section width, which can be found on the side of your tire. 

The lower the Aspect Ratio, the shorter the sidewall and the wider the shoulder-to-shoulder width.

Overall Diameter is the width (in inches) from top to bottom of the tire.

Rim Diameter is the width (in inches) from top to bottom of the wheel, which can be found on the side of your tire.

Rim Diameter is the width (in inches) from top to bottom of the wheel, which can be found on the side of your tire.